Which invention was Cyrus McCormick famous for developing?

Cyrus McCormick is most famous for developing the mechanical reaper, an invention that revolutionized agriculture in the 19th century. The mechanical reaper significantly increased the efficiency of harvesting crops by allowing one farmer to do the work that previously required multiple laborers with scythes. This advancement led to greater agricultural productivity, particularly in the grain-producing regions of the United States. As a result, it played a crucial role in the market economy of the time and supported the westward expansion by facilitating the growth of large-scale farming operations.

The other inventions listed, such as the steam engine, telegraph, and spinning jenny, were important innovations, but they are more closely associated with other inventors or industries. The steam engine is linked to figures like James Watt and played a key role in the transportation and industrial revolutions. The telegraph, which revolutionized communication, is attributed to Samuel Morse, while the spinning jenny, a significant development in textile manufacturing, was invented by James Hargreaves. Each of these inventions transformed their respective fields but is not connected to Cyrus McCormick.